MALADAPTIVE PLASTICITY
FOLLOWING INJURY
Although neuronal plasticity is
usually adaptive in that it supports beneficial
responses and behaviors, excessive
plasticity has also been implicated in the
pathogenesis of neurological disorders.
Two examples of maladaptive plasticity
are use-dependent dystonia and phantom
limb pain. Musicians or writers can
develop involuntary dystonic co-contraction
of the muscles in the hands and
fingers over time after prolonged use
[Quartarone et al., 2006]. Physiological
studies of somatosensory cortex in these
patients indicate that the representation
of individual digits is less segregated on
both sides of the brain than in control
subjects. Studies utilizing TMS also
showed that their motor cortex was
more easily activated than cortex in
controls and the response to focal stimulation
of motor responses in the hand
muscles was less spatially restricted than
in controls. These results suggest that
individuals who develop hand dystonia
related to overuse have abnormal cortical
plasticity that may in some cases be
due to some underlying genetic predisposition.
Certain cases of phantom limb
pain following amputation reflect a similar
phenomenon occurring in sensory
cortex [Flor, 2008]. Phantom limb pain
occurs more commonly after amputation
in adults than children and is associated
with reorganization at multiple
levels in the spinal cord, brain stem,
thalamus, and cerebral cortex. Studies
in individuals with arm amputation
indicate that pain is associated with
reorganization of sensory cortex so that
the representation of the mouth area
moves into the area normally occupied
by the arm. Patients with phantom pain
have also been shown to have enhanced
excitability in the sensory cortex.